This invention relates to a process and structure for protecting objects from electrical discharges by employing a coating of a conductive, ablative material such as graphite.
There are certain well established methods used for protecting objects, especially conductive ones, from making contact with other electrified objects or from being contacted by electrical discharges.
In relatively low voltage electric fields, discharges in air do not generally occur when a metal conductor is protected by small thicknesses of insulating material such as cotton or silk fabric, rubber or plastic coating, enamel or paint, resin, and porcelain. Insulation of this type is usually satisfactory for applications such as exterior and interior low voltage electric house wiring, and telephone, radio and television lead wires.
In relatively high voltage electric fields, however, it is generally not advantageous, unless a conductor has to pass in close proximity to another conductor, to cover either conductor with sufficient insulating material to prevent breakdown, for the required thickness would be impractical. Instead, the usual practice is to support an entirely uncovered high voltage conductor by a stand-off insulator so that the distance from neighboring objects, including the earth and storm clouds, exceeds the breakdown distance of the electric field in air. In fact, the addition of only a small amount of insulating coating such as was just described for low voltage insulation is generally considered to be deleterious, for if the coating is punctured by the discharge, the damaged insulation may subsequently act as a source of corona discharge and precipitate further breakdown.
It was pointed out by the inventor (see pages 430 and 431 of the book entitled Problems of Atmospheric and Space Electricity, edited by S. C. Coroniti, McGraw-Hill, 1965, and also pages 1365 to 1375 of the publication Journal of Geophysical Research, Volume 68, 1963) that, based on analyses of lightning strikes, the greatest contribution to damage from an intense electrical discharge, such as a lightning return stroke, appears to arise from the electronic heating of the conductor precisely at the conductor surface where the lightning stroke attaches.
Based upon such analyses, and upon many other experiments, various techniques and structures have been developed for preventing or minimizing the damage resulting from intense electrical discharges, especially lightning strikes. The literature, including the patent literature, is replete with examples of such techniques and structures. Adequate protection of high voltage lines, airplanes, buildings and many other objects exposed to such discharges is essential. Yet substantial damage continues to be produced by such discharges, even to objects thought to be adequately protected.
The present invention is directed to providing adequate protection for objects subject to electrical discharges, and to preserving conductors and metals which are now normally exposed without protection to high voltage fields that may cause breakdown, arcing or corona in air.
Adequate protection requires protection from both the electrical and mechanical effects of a discharge. Electrical protection includes appropriate fuses, high voltage shunts and other structures which, if designed with care, are satisfactory and prevent appreciable electrical damage. Protection against the subsequent mechanical effects of an intense electrical discharge, however, has proven considerably more difficult. Inspection and analysis of objects thought to be adequately protected often reveals that failure resulted from melting of the object at the point of impact, which not only weakened (usually to the point of failure) the object but also resulted in a discontinuity giving rise to arcing or corona discharge thereby precipitating further discharges to this weakened area.
To achieve adequate protection, this invention contemplates coating, or otherwise applying to the object, a material both capable of conducting the electrical current stroke and of dissipating away the localized heat generated by the stroke, preferably without melting. Rather, the material will vaporize under the localized heat imparted to it. The coated material has therefore preferably a high heat capacity to absorb heat energy directly by sublimation. A material of high latent heat of vaporization and of high melting point has the capability of dissipating a large amount of heat while maintaining its integrity as a solid. A material which is both a conductor and a heat absorber might be generally referred to as a conductive ablator. Graphite (an allotropic form of the element carbon) is an excellent example of a conductive ablator, for not only is it a moderately good conductor, especially at high temperatures, but it is also an excellent absorber of heat. Graphite remains a solid at a temperature of at least 3500.degree.C and, at atmospheric pressure, probably vaporizes directly at temperatures above this without passing through a liquid phase. Other examples of a conductive ablator include tungsten, tantalum and titanium, which although costly and subject to melting at high temperatures nevertheless are capable of conducting a current stroke and dissipating considerable heat. Thus when an object protected by such a conductive ablator is subjected to an intense electrical discharge, for example a lightning stroke, the resulting electrical current will be conducted while the resulting and substantial direct surface heating will be dissipated by evaporation of the ablator. Considering a preferred embodiment, evaporation of a graphite coating in still air at temperatures of between 2000.degree.C and 3600.degree.C will result in an average loss in thickness of graphite on a surface parallel to the basal (slip) planes of between 0.005 and 0.001 inch per minute. In use, the actual loss of coating thickness even during an intense lightning strike to the surface will be rather minor, and only a thin coating is required for adequate protection.
In the following detailed description of the invention the process and structure for protecting objects from electrical discharges will be described, and a number of examples presented, in connection with the following drawings.